The present invention relates to a conductor composition used for low temperature cure/dry but required high thermal resistance, such as circuit lines on the plastic board, termination of resistor/capacitor/inductor, or tantalum capacitor conductive coating material.
The so-called “fired type” conductive paste formulations have been applied to ceramic substrates by printing method to make circuit components including external electrodes for the chip resistor, the chip capacitor or the chip inductor. For flexible substrates, polymer type conductive paste based on organic resin have been instead of the fired type. These pastes, too, are typically applied to the substrate using printing techniques. Since proper viscosity of the conductive paste or composition is essential for application by a printing technique, polymer based pastes are either made using thermosetting or thermoplastic resins. Thermosetting resins are applied as monomers or oligomers having suitable viscosity for paste application. Then on heating the polymer will crosslink or polymerize to deliver the desired properties for the cured paste. On the other hand thermoplastic resins may be used which soften in a solvent for application and dry to a hard film after printing.
In case of using the organic resin, thermoset plastic resin such as epoxy resins or phenolic resins are used for high heat resistant application. The heat applied in to cure thermoset resin based conductive paste, is usually that required to fully polymerized paste. Low cure temperature thermoset resins based conductive pasts usually require that the paste itself should store in refrigerator or freezer so that curing will not begin until before the desired time. Pastes that can be safely stored at room temperature require higher curing temperatures. Problems arise in application of the uncured pastes and in the quality and use properties of the cured pastes if the storage temperature, the cure start temperature and the temperatures tolerance of the substrate are not properly selected.
A second option for polymer based conductive paste formulations is the use of thermoplastic resin such as acrylic resin or polyester resin. Thermoplastic resin based conductive pastes use is almost independent of drying temperature. The reason is the thermoplastic resin do not crosslink or polymerize in the paste system because these resins are already long chain polymer, and the chemical polymerization. Heating of the printed paste is not necessarily required unless to accelerate the evaporation of the solvent. Because the selection of drying conditions includes low temperature and the selection of resin for the paste and the substrate composition includes more possibilities, thermoplastic resins are widely used for preparing the polymer or resin based conductive paste compositions.
Additionally, the shelf-life of the thermoplastic resin based conductive polymer is longer than thermoset plastic resin based paste. However, the thermoplastic resin based paste, generally have poorer thermal resistance compared to the thermal resistance of thermoset resin based paste.